EXPERIMENTS ILLUSTRATING THE USE OF APPARATUS 19 



coils are again parallel. This principle is embodied in Bowditch's 

 induction coil. 



8. Connect up the battery with the terminals, t 3 , fi, of the in- 

 duction coil (as in fig. 23), introducing a simple key into the 

 circuit. Set the Neef's hammer in vibration. The electrodes from 

 the secondary coil are to be applied to the tongue, and the distance of 

 the secondary from the primary coil found at which the induced 

 shocks can just be felt. Determine that these are the break shocks 

 by raising and lowering the hammer by the hand, and thus slowly 

 making and breaking the primary circuit (the mercury key being 

 closed). 



9. Detach one of the wires of the electrodes from the secondary 

 coil so that only one electrode is connected with that coil. Slide the 

 coil home. Pass a strong current through the primary coil and set 

 Neef's hammer going as in the last experiment. It will be found that 

 shocks are faintly felt by the tongue, although only the one electrode 

 is in connection with the secondary coil and the secondary circuit is 

 broken (unipolar induction). 1 It is on account of this possibility of 

 stimulating through only one pole that a simple key is never used in 

 the secondary circuit, but always a short-circuiting key, which is 

 introduced in the manner shown in fig. 22. No shocks can pass to the 

 electrodes when the key is closed, since the coil is then short-circuited ; 

 only when the key is open are the shocks conducted to the electrodes. 

 On the other hand, in the primary or battery circuit a simple key is 

 always used ; were a short-circuiting key placed here the battery 

 would rapidly run down. 



10. Connect up a battery with the induction coil, using Helm- 

 holtz's modification (fig. 24). As in experiment 8, find the distance 

 of the secondary from the primary coil at which the induced shocks 

 can just be felt on the tongue, and determine that the make and break 

 shocks are now nearly equal by raising and lowering the spring by the 

 hand. Both are markedly diminished. 



1 The explanation of this is that the body acts as a condenser which becomes 

 charged and discharged through the electrode applied to the tongue. 



